The frequency stability is usually determined by the requirements of the application and will subsequently determine the type of crystal oscillator that will be needed. The temperature range over which the oscillator must operate is a major factor in determining the stability that can be achieved.
Crystal Oscillator Types
Simple Crystal Oscillator (XO): This is the most basic type where the stability is totally determined by the inherent characteristics of the crystal resonator itself. The higher frequency crystals in the MHz range are fabricated from a quartz bar in such a way as to provide a relatively stable frequency even though the ambient temperature may vary as much as -55°C to +125°C (-67°F to +257°F). A stability of ±25ppm is achievable with a properly cut quartz crystal even over this wide of a temperature range. This is a substantial improvement over other passive resonators such as an LC tank circuit which may change 1% or more (10,000ppm). But even 25ppm is not good enough for some applications so additional measures must be employed.
Temperature Compensated Crystal Oscillator (TCXO): If the inherent frequency versus. temperature stability of the quartz crystal is not adequate for an application, a temperature compensated unit may be employed. A TCXO uses a temperature sensing device along with circuitry which generates a voltage curve which is the exact inverse of the crystal over the temperature range and ideally cancels out the drift of the crystal. Typical stability specifications for a TCXO range from less than ±0.5ppm to ±5ppm depending on the type of TCXO and the temperature range.
Oven Controlled Crystal Oscillator (OCXO): For some applications the frequency versus temperature stability of a TCXO will still be inadequate. In these cases, an OCXO may be called for. As the name implies, an oscillator with an oven heats the crystal to an elevated temperature which is controlled so that the temperature of the crystal remains stable even though the ambient temperature may vary widely. Since the temperature of the crystal and the sensitive portions of the oscillator see very little variation, the frequency versus. ambient temperature stability is substantially improved. The stability of an OCXO may be as tight as 0.001ppm over the ambient temperature range. This improved stability, however, comes with the cost of increased power consumption in order to supply the heat to the oven. A typical OCXO may require from 1- to 5-W of power to maintain the internal temperature. A warmup period after turn on is also needed to wait until the temperature and frequency have stabilized, typically from 1 minute to greater than 10 minutes depending on the type of unit.